The present invention relates to optical crossconnect switches and, more particularly, to a large Nxc3x97N arrangement of binary trees suitable for optical crossconnect switches.
In high capacity optical networks, an essential device is the Nxc3x97N crossconnect switch. The function of this device is to provide at each node full connectivity among several incoming fibers, each carrying several wavelength channels. The switch must be nonblocking [1-11], and it must be fast and efficient. (Note in this specification, a reference to another document is designated by a number in brackets to identify its location in a list of references found in the Appendix.) These properties can be realized with minimal loss and crosstalk by using the classical crossbar construction, consisting of binary trees of 1xc3x972 and 2xc3x971 elements [2]. However this arrangement becomes impractical for large N, because of its large number of elements given by 2N (Nxc3x971). Because the number of elements increases quadratically with N, the Nxc3x97N crossconnect switch rapidly becomes too large for large N. For example, the number of elements for N=64 and 128 are, respectively, 8064 and 32512. With the ever-increasing capacity of optical networks there is continuing need for even larger Nxc3x97N crossconnect switches.
What is desired is a large Nxc3x97N crossconnect switch that minimizes the number of elements without a significant increase in loss and crosstalk.
In accordance with the apparatus of the present invention, a dilated Nxc3x97N optical cross-connect switch suitable for large N is disclosed which exhibits good efficiency (small element count) realized with low values of depth (low loss) and crosstalk. As compared to the classical crossbar switch arrangement, both width and element count are substantially reduced, without substantially increasing depth and crosstalk. The present arrangement can be realized, in its simplest form, by using six levels of binary trees of 1xc3x972 and 2xc3x971 switching elements. The first two levels form the input stage, the third and fourth levels form the center stage, and the fifth and sixth levels form the output stage. In each of the input, center, and output stages, the odd number level is formed as a binary tree of 1xc3x972 elements and the even numbered level is formed as a binary tree of 2xc3x971 elements. Each stage is formed by directly joining together the 1xc3x972 element binary tree with the 2xc3x971 element binary tree. Similarly, adjacent stages are joined together by directly connecting together pairs of trees. In one embodiment, depth is substantially reduced by using a symmetric arrangement of one or more 2xc3x972 elements located in the vicinity of each junction between adjacent stages. Using the present arrangement, the number of elements needed for a Nxc3x97N cross-connect, where N=64, 128 is, respectively, approximately 4500, 13500, as compared to about 8000, 33000 required using two stages of binary trees as in the classical crossbar arrangement. The arrangement also features reduced depth obtained by a Clos type construction. Crosstalk is of order two, and it causes a maximum of three components transmitted to each output port.
More particularly, my invention is directed to an Nxc3x97N nonblocking optical switch for providing a connection between any of N inlets and any of N outlets, where N is an integer, the Nxc3x97N switch comprising (1) an integer number p of layers, each formed by a distinct Nxc3x97N arrangement; (2) an integer number N of 1xc3x97p input space switches, each input of each of the N input space switches connects to a different one of the N inlets, and each input space switch connects to all layers; (3) an integer number N of pxc3x971 output space switches, each output of each of the N output space switches connects to a different one of the N outlets, and each output space switch connects to all layers; (4) where each layer consists of three stages, the first and third of which consist of mxc3x97m switches and the second stage consists of       N          m      2        xc3x97      N          m      2      
nonblocking switches; (5) where each mxc3x97m switch is capable of forming at least one path from any of its m input ports to any of its m output ports, and each mxc3x97m of the first stage is connected to each mxc3x97m switch of the third stage via one particular switch of the second stage; and (6) where the number p of layers satisfies pxe2x89xa72mxe2x88x921.
According to another aspect of my invention, an Nxc3x97N nonblocking optical switch provides a connection between any of N inlets and any of N outlets, where N is an integer, the Nxc3x97N switch comprising, in its simplest form, three stages respectively consisting of mxc3x97p ,             N      m        xc3x97          M      m        ,
and pxc3x97m switches, where m is an integer divisor of N an p is an integer satisfying pxe2x89xa72mxe2x88x921. Each stage consists of two levels of trees, and the complete arrangement comprises, (1) six levels of trees formed by 1xc3x972 or 2xc3x971 switching elements arranged in an input stage, a center stage and an output stage; (2) where each even level of trees is formed by 1xc3x972 elements, so that each tree has one input port, which is formed by the 1xc3x972 element at the root of the tree, and it has many output ports formed by the output 1xc3x972 elements corresponding to the leaves of the tree; (3) where each odd level of trees is formed by 2xc3x971 elements, so that each trees is reversed, thus having one output port corresponding to the root and many input ports corresponding to the leaves. The input stage consisting of mxc3x97p switches including a first level and a second level of binary trees, each tree of the first level having its input port connected to a different one of the N inlets and having each output port connected to a different tree of the second level of trees, each output port of the second level of trees being connected to a different tree of the center stage. The center stage consisting of       N    m    xc3x97      M    m  
switches, including a third level and a fourth level of binary trees, each tree of the third level having each output port connected to a different tree of the fourth level of trees, each output port of the fourth level of trees being connected to a different tree of the output stage. The output stage consisting of pxc3x97m switches including a fifth level and a sixth level of binary trees, each tree of the fifth level having each output port connected to a different tree of the sixth level of trees, each output port of the sixth level of trees being connected to a different one of the N outlets.
An important feature of the above arrangement is that consecutive stages are joined together by directly connecting together the roots of the trees on either side of the junction. As a consequence, both depth and number of elements can be reduced by replacing the 1xc3x972 and 2xc3x971 elements close to the junction with a smaller number of 2xc3x972 elements.